1. Field of the Invention
The present invention relates to a developing device for use in an image forming apparatus such as a laser printer, etc., for developing an electrostatic latent image by supplying toner to the electrostatic latent image formed on an outer peripheral surface of a photosensitive drum and transferring the image developed on the surface of the photosensitive drum onto a sheet, and particularly to a developing device in an image forming apparatus, capable of providing a toner layer which is supplied on a surface of a developing roller so as to be uniform in thickness by making a surface of a developing roller into a rugged surface provided with convex portions having a height approximately equal to a diameter of a toner particle to be supplied thereon, thus enabling to evenly charge the toner layer to form a resultant image, i.e., a visual image with uniform image density.
2. Description of Related Art
Regarding conventional developing devices for use in image forming apparatuses such as laser printers, etc., there have been proposed various types of the devices, which are in general constructed of a toner storing member including a toner cartridge for storing therein toner, a toner supply roller for supplying toner from the toner storing member, and a developing roller for developing an electrostatic latent image on a photosensitive drum by supplying the toner provided from the toner supply roller onto the image. One embodiment of the developing device will be explained with reference to FIG. 6. FIG. 6 is an explanatory view showing schematically a main construction of the developing device in the prior art.
In FIG. 6, the developing device has a toner cartridge 100 which accommodates therein toner and is provided with an opening for toner supply at an almost center in its width direction. This toner cartridge 100 is provided therein with an agitator 103 for agitating toner to supply same into a developing chamber 102 side through a toner supply port 101. A frame F of the developing device is provided with an opening for toner supply positioned correspondingly to the toner supply opening of the toner cartridge 100. Those openings of the toner cartridge 100 and the frame F form the toner supply port 101 in combination with each other. Inside the developing chamber 102 constructed of an upper frame F1 and a lower frame F2 of the frame F, a toner supply roller 104 is arranged rotatably in a lower frame F2 side, for supplying the toner supplied through the toner supply port 101 to a developing roller 105.
Here, the developing roller 105 used in the conventional developing device will be explained with reference to FIG. 7. FIG. 7 is an enlarged view showing a part of the developing roller in the prior art.
This developing roller 105 is usually formed of silicone rubber because it is excellent in a charging performance to frictionally charge toner although it is not very high in abrasion resistance. The surface of the developing roller 105 is subjected to a rubbing process so as to become about 3-6 .mu.m at ten points average roughness. When the rubbing process is applied to silicone rubber to obtain the above surface roughness, however, the rubbed surface of the developing roller 105 is actually rugged, having concave portions 105A with a depth X being about several ten .mu.m and convex portions 105B with a cycle, i.e., a distance between the convex portions 105B being about some hundreds .mu.m, as shown in FIG. 7. This is due to that the silicone rubber is coparatively soft material. At this time, the diameter of a toner particle T carried in each concave portion 105A is generally about 5-10 .mu.m.
Furthermore, on an internal wall of the upper frame F1, above the developing roller 105, a blade 107 is fixedly secured with a fixing element 106, whereby regulating a thickness of the toner layer supplied on the surface of the developing roller 105. This developing roller 105 is also arranged in contact with a photosensitive drum 108. On the peripheral surface of the photosensitive drum 108 is formed an electrostatic latent image by an image exposure device not shown which performs a scanning operation with a laser beam in accordance with image data. The developing roller 105 supplies toner to the electrostatic latent image formed on the peripheral surface of the photosensitive drum 108 to develop the image. The image developed on the surface of the photosensitive drum 108 is then transferred onto a sheet fed from a sheet feeder not shown, forming the resultant image thereon.
On the surface of the developing roller 105 used in the conventional developing device, even if the developing roller 105 is a new article, however, formed are concave portions 105A each having a depth X of about several ten .mu.m and convex portions 105B having a cycle Y between those convex portions 105B of some hundreds .mu.m. Therefore, it is not too much to say that the surface of the developing roller 105 is in a very rough condition. The diameter of a toner particle T held in the concave portions 105A is usually 5-10 .mu.m, so that there is a large difference in size between the depth X of the concave portions 105A and the toner particle T. Accordingly, the toner particles T tend to be held inside the concave portions 105A as shown in FIG. 7.
When the toner particles T are carried on the surface of the developing roller 105, therefore, the toner particles T largely tend to be held inside the concave portions 105A rather than on the convex portions 105B, so that the toner particles T supplied on the surface of the developing roller 105 form partially a thin layer and a thick layer. This causes a problem that it is extremely difficult to hold the toner particles T as a layer with a uniform thickness on the surface of the developing roller 105.
By the way, it will be conceivable that size of the concave portions 105A on the developing roller 105 gradually changes in small size while being used for image forming. This is due to the same reason described above that the silicone rubber is comparatively soft material, thus, the surface condition of the developing roller 105, in which the size of the concave portions 105A is big size when the developing roller 105 is a new article, is remarkably changed in comparison with the surface condition of the developing roller 105, in which the size of the concave portions 105A is small size after the developing roller 105 is used for a predetermined time. Therefore, it concludes that the layer of the toner particles T with uniform thickness cannot be formed on the surface of the developing roller 105 over the period passing until the developing roller 105 is used for a predetermined time from a start of use thereof.
As mentioned above, when the layer of toner particles T is uneven in thickness on the developing roller 105, it is impossible to uniformly charge the toner T, the charging being performed by a friction between a supply roller 104 and the developing roller 105 and between a blade 107 and the same, and a voltage applied to the developing roller. Consequently, it causes that the toner is unevenly supplied to the electrostatic latent image formed on the photosensitive drum 108 in a development operation on the image. Thus, a resultant image can not be formed with uniform image density.